Production of discontinuous, conducting coatings upon insulating surfaces



Feb. 12, 1952 s. o. DoRsT 2,585,752 l PRODUCTION OF DISCONTINUOUS,CONDUCTING COATINGS j UPON INSULATING SURFACES :f Filed May 2e, 1948JTA/VLE 7 0 DoRT JNVENTOR.

Patented Feb. 12, 1952 PRODUCTION OF DISCONTINUOUS, CON- DUCTINGCOATINGS UPON INSULATING SUR-FACES Stanley O. Dorst, North Adams, Mass.,assignor to Sprague Electric Company, North Adams, Mass., a corporationof Massachusetts Application May 26, 1948, Serial N o. 29,317

6 Claims. (Cl. 2925.42)

My invention relates to a process for producing discontinuous,conducting coatings upon surfaces composed of heat resistant, insulatingmaterials. More particularly, it concerns a process whereby separate,coated metal electrodes are formed upon the surface of a heat resistant,insulating body such as is used, for example, in the production ofceramic condensers and printed circuits.

Ceramic condensers, especially those of the tubular type, are of growingimportance in the electrical field, due to the development of highdielectric constant ceramic materials which can be processed withoutloss of their desirable electrical characteristics. One typicalcondenser is made by extruding a cylindrical tube of a semi-pastyceramic mix, drying and firing 'the extruded tube, before or aftercutting it into suitable lengths. Each length of tubing is thereafterprovided with an outer and an inner electrode, which are highlyconducting and integral with the surface of the ceramic. One of the mainproblems encountered in providing such electrodes arises from the factthat the two electrodes applied to the ceramic surface must be insulatedfrom one another, Le., the conducting coating upon the ceramic tube mustbe divided.

According to one method developed for applying separate, coatedelectrodes, a silver lacquer is sprayed or painted only on thoseportions of the ceramic tube upon which the electrodes are desired by acareful masking operation. Thereafter, the silver layer is fired atelevated temperatures, to remove the binder and give an adherent silvercoating on the ceramic. The electrodes thus produced are satisfactoryfor use without further treatment. However, the silver layer so formedis relatively ,thick and correspondingly expensive.

According to another method, the ceramic body is coated with aninsulating lacquer on those areas which are not`to be provided with ametal coating. Thereafter, the uncoated surface is provided with a layerof chemically reduced silver, which layer is in turn electroplated withcopper or some other readily solderable material. The insulating lacqueris then removed with a suitable solvent, and terminal wires are solderedto the electroplated layer.

The last mentioned process requires less silver than the one firstdescribed, but at the same time is not satisfactory, because of thedifficulties encountered in plating the initial silver layer. Theinexpensive barrel plating process is unsuitable for this purpose, sinceit is difficult to 2 make electrical contact to both of the silveredareas of each ceramic body. As a result, many ceramic condensersproduced by this process come out with only one electroplated electrode.Another difliculty is that chemically reduced silver is deposited uponthe insulated lacquer from which it may be transferred to the underlyingceramic surface, when that lacquer is removed with solvent. Thisresidual silver may cause a low re- -sistance path between the twoelectrodes.

It is an object of the present invention to overcome the foregoing andrelated disadvantages of the prior art procedures. A further object isto produce discontinuous, solderable metal coatings on heat resistant,insulating materials. A still further object is to produce separate,coated metal electrodes on ceramic bodies by a simple, inexpensive andnovel process. Additional objects will become apparent from thefollowing description and claims.

These objects are accomplished in accordance with my invention by firstproviding the entire surface of the insulating body with a thin layer ofa highly conducting metal, such as silver, platinum and palladium.Selected portions of this metal layer are then masked with an imperviousmaterial. Thereafter, the exposed portions of the metal layer areelectroplated with a conducting metal, preferably a solderable one. Thenthe masking material is removed, and the entire body is treated with aselective solvent, which serves to remove those portions of theoriginal, highly conducting metal layer, that were .previously masked bythe impervious material.

Thus, a discontinuous, conducting coating of an electroplated metal isproduced upon the surface of the insulating body.

In a more restricted sense my invention in concerned with a process forproducing separated electrodes on ceramic bodies, which comprises thesuccessive steps of providing the entire ceramic surface with a thinlayer of chemically reduced silver, coating those portions of the bodysurface that are to separate the electrodes with an insulating lacquer,electroplating the exposed, reduced silver layer with a solderablemetal, removing the insulating lacquer with a solvent, and then dippingthe entire bodies in a molten solder bath. In one of its preferredembodiments, this invention is concerned with a process for producingseparated electrodes on a ceramic body, which comprises providing theentire surface of said body with a layer of chemically reduced silvernot greater than about 0.0001" in thickness, coating those portions ofthe body surface which are to separate'the electrodes with an insulatinglacquer, electroplating the exposed silver layer with copper to athickness between about 0.0005" and 0.002", removing the insulatinglacquer with a solvent, applying terminals to the electrodes, anddipping the ceramic body in a bath of molten solder selected from theclass of those containing lead, tin, bismuth, zinc, copper, cadmium andtheir alloys.

The initial thin metallic layer is deposited over substantially theentire surface of the insulating base by well known means, such asspraying. sublimation, sputtering, photochemical reduction, etc. Aparticularly satisfactory method is the so-called chemical reductionmethod, wherein a silver salt is reduced to produce the silver particleson the surface. By this process, the insides of tubes, etc. may beprovided with a conducting coating. Provision of a conducting coatingover substantially all of the surface is desirable, as hereinafterexplained.

Metals other than silver may be deposited For example, platinum andpalladium are suitable.

Thereafter, those portions of the surface which are not to serve aselectrodes are masked with a wax, resin, Scotch tape or other imperviousmaterial. Resin lacquers are often used for this purpose.

Since the initial layer is very thin and relatively fragile, it iscustomary to electroplate the electrodes with copper, zinc or some otherreadily solderable material. The coated base may be subjected to abarrel plating process, wherein a. large number are plated in a singleoperation.

y'Electrical contact between adjacent bases need be made at but a singlepoint, since the initial conducting layer covers the entire surface.Electroplating will occur only on the exposed metal layer sections, thusproducing two or more electrodes as desired.

'I'he masking materials separating the electroplated portions may thenbe removed, leaving thereunder a conducting metal layer whicheffectively short circuits the two or more electroplated electrodes.These areas are removed in accordance with my invention by treatmentwith a selective solvent. By this term, I mean a material which willremove the initial conducting layer preferentially or at a rate suchthat the latter will be entirely removed without appreciably affectingthe electroplated layers. A number of selective solvents which may beemployed in accordance with my invention are described in the followingparagraphs.

Certain metallic solvents are suitable. Various solders, such as alloysof lead, tin, bismuth, zinc. copper, silver and cadmium, are useful,although the melting point thereof may be appreclably less than themelting point of the conducting layer metal. For example, a chemicallyreduced layer of silver will dissolve in a solder of 50% lead-50% tincontent held at 260 C. The solders will not wet the ceramic surfacesafter dissolving the thin metal layer. Other metals and alloys may beused, mercury being representative. Solders are particularly desirablesince the removal of the inter-electrode areas may be combined with thesoldering of terminal elements to the various electrodes.

The selective solvent may be based on an electrolytic treatment, wherebythe metal passes into solution in the ionic state. It is possible todissolve the initial layer preferentially or at a higher rate than theelectroplated layer by suit- The following description will beparticularly` concerned with this selective solvent and process, but itis to be understood that the other selective solvents referred to abovemay be employed on.

a chemically reduced or otherwise produced initial layer.

Theventire surface of the insulating body is provided with a chemicallyreduced silver layer generally of a thickness less than about 0.0002".Those portions of the body surface that are to separate the conductingcoating, are then masked with a layer of material, generally insulatingand non-pervious, such as a nitro-cellulose lacquer. Thereafter, theexposed and uninsulated silver surfaces are electroplated with a metalor alloy, copper being representative of the electroplated metal. Thisplating step can be conducted in a barrel or other massive arrangement,since the insulating bodies may be dumped freely into the electroplatingbath, and need contact each other only at a single point where thesilver layer is exposed. One such body must, of course, make contactwith the cathode terminal of the plating assembly.

The plated body is then treated with a solvent and/or with heat toremove the masking coating. leaving the unplated silver layer exposed.At this point, it is possible mechanically to attach terminal elements,such as tinned copper wires. to the electroplated electrodes. The wholebody is then dipped in a molten solder bath. During this step, theelectroplated surface and terminal contact become coated with a solderlayer, thus permanently connecting any such terminal elements, while theexposed silver layer is dissolved in the molten solder bath, leaving theunderlying surface of the insulating body completely uncoated and,therefore, non-conducting. In this manner, the diiliculties attendantwith prior processes employing the so-called chemical silver process,are completely overcome.

My invention will be further discussed with reference to the appendeddrawings in which Figures 1 through 5 represent a ceramic condenser bodyat various points during the processing sequence of the invention.

More specifically, Figure 1 shows a tubular ceramic condenser body I0,the entire surface of which has been provided with a chemically reducedsilver layer Il. The thickness of this layer and of the others is showngreatly exaggerated for purposes of clarity. In Figure 2 a coating ofmasking lacquer l2 has been applied to those portions which are toseparate the Y condenser electrodes. The lacquer or other maskingmaterial may be applied with a brush, spray device, marking wheel or byother known methods.

Figure 3 shows the metal electrodes I3 and Il which are electroplated onthe exposed surfaces of .the silver layer Il. This plating may beaccomplished by the barrel process, in which the bodies to be plated aredumped en masse into a plating barrel containing the appropriate platingsolution.

Figure 4 shows the body. after the lacquer coating I2 has been removed,leaving portions of the silver layer Il exposed, and short-circuitingthe condenser electrodes I3 and I4. Terminal wires I5 and I8 are shownwrapped around the outer surface of electrodes I3 and I4, respectively.These terminal elements are generally of tinned copper.

Figure 5 shows the condenser after it has been dipped in a molten solderbath. A solder layer I8 covers electrode I3 and bonds terminal I5thereto, while a solder layer I1 coats electrode I 4 and bonds terminalI6 thereto. The molten solder bath has dissolved and completely removedthe exposed silver surface, as indicated at I9 and 20, leaving theceramic body surface uncoated.

The finished condenser shown in Figure 5 may be dipped in wax, coatedwith an insulating lacquer or protected by a molded plastic casing, inthe usual manner.

While the example illustrated and specifically described above is atubular ceramic condenser, the invention may also be applied to manyother insulating bodies in many different shapes and configurations.Ceramic materials which may be provided with electrodes in accordancewith the invention include steatite, titanium dioxide, alkali metaltitanates, various glasses, such as the borosilicate glasses, quartz,etc. Other insulating materials include the heat resistant organic orsemi-organic resins, such as the polysiloxanes. polyhaloethylenes, etc.The invention is particularly applicable to polytetrahaloethylenes, suchas polytetrafluoroethylene, polytetrachloroethyly ene andpolychlorotriuoroethylene. These resins do not soften at temperaturesbelow the melting points of many of the coating soldersreferred toherein, and are extremely resistant to the usual solvents. Theinsulating bodies may be fiat, tubular, rod-shaped and in sheet form, aswell as in special congurations for special purposes.

The chemically reduced silver layer may be applied by any suitablemethod. One highly satisfactory procedure is to dip the insulating bodyin a mixture of 275 parts of a silvering solution containing 20grams/ml. of AgNOa, 19 grams/ l. of KOH and 21/ml./ 1. of NHiOH conc.,and of 34 parts of a reducing solution containing 100 grams/ 1. ofdextrose and 158 ml. of CHaOH. The insulating body is allowed to remainin the mixed solutions for about 5 minutes. After rinsing and drying,the body will have a silver layer about 0.0001" thick thereon.

The electroplating may be done with any suitable conducting metal.Copper is preferred, since it is an excellent electrical conductor and.when clean, readily solderable. Other suitable metals include nickel,cadmium, tin, lead, etc. One satisfactory copper electroplating processemploys a bath containing 120 grams of copper cyanide, 135 grams ofsodium cyanide and .30 grams of NaOH per liter of water. The bath isheld at lI-80 C., and the plating is carried out at a current of 4100amperes per square foot to be plated, until the desired thickness isbuilt up. A copper thickness of 0.001" is satisfactory for the presentpurposes, although thicknesses as low as about 0.0002" may be used.

The solder bath is held above the melting point of the particular solderto be used, generally between about and 100 C. above its melting point.The solder should dissolve the silver, and for this purpose, copper,lead. tin, zinc, bismuth, cadmium and silver solders may be used. A suitavoiding a separate step.

It is apparent that the invention is applicable to the preparation ofnumerous electrical and mechanical devices which require adiscontinuous, conducting coating on an insulator. The process of theinvention leads to durable, adherent, metal coatings, separated bywholly uncoated portions of the insulating surface.

As many different embodiments of this invention may be made withoutdeparting from the spirit and scope hereof, it is to be understood thatthe invention is not limited to the specific embodiments hereof, exceptas defined in the appended claims.

I claim:

1. A process for producing discontinuous, conducting coatings upon thesurfaces of insulating bodies, which comprises providing the entiresurface of the insulating body with a thin layer of a metal of the classconsisting of silver, platinum and palladium masking portions of saidlayer with an insulating coating, electroplating the exposed portions ofsaid layer with a conducting metal of the class consisting of copper,nickel, cadmium, lead and tin, removing the insulating coating, anddipping the entire surface in a bath of molten solder composed of analloy of at least two metals selected from the group consisting of lead,tin, bismuth, zinc, copper, silver and cadmium, until the newly exposedportions of the thin layer have been dissolved by the solder.

2. A process for producing separated electrodes upon the surfaces ofceramic bodies, which comprises providing the entire ceramic surfacewith a thin layer of silver, coating those portions of the body surfacewhich are to separate the electrodes with an insulating lacquer,electroplating the exposed portions of the silver layer with aconducting metal of the class consisting of copper, nickel, cadmium,lead and tin, removing the insulating lacquer with a solvent, and thendipping the entire surface in a bath of molten solder composed of analloy of at least two metals selected from the group consisting of lead,tin. bismuth, zinc, copper, silver and cadmium, until the newly exposedportions of the thin silver layer have been dissolved by the solder.

3. A process for producing separated electrodes on a ceramic body, whichcomprises providing the entire surface of said body with a layer ofchemically reduced silver, not greater than about 0.0001 in thickness,coating those portions of body surface which are to separate theelectrodes with an insulating lacquer, electroplating the exposed silverlayer with copper to a thickness between about 0.0005" and 0,002,removing the insulating lacquer with a solvent, and dipping the ceramicbody in a bath of molten solder composed of an alloy of at least twometals selected from the group consisting of lead, tin, bismuth, zinc,copper, 'silver and cadmium, until the newly exposed portions of thethin silver layer have been dissolved by the solder.

4. A process for producing ceramic condensers, which comprises theprocess dened in claim 2 wherein tinned copper terminal wires are woundabout the electroplated portions that are to serve assuma n W aselectrodes, before the ceras Ilcuody is flipped in the molten solderbath, whereby the electroplated portions and the terminal wires aresimultaneously coated with a solder layer and thus permanently connectedto each other.

5. A process for producing electrical devices, which comprises theprocess defined in claim 1, wherein terminal elements are mechanicallyattached to the electroplated portions, before the insulating body isdipped in the molten solder bath. whereby such terminal elements aresimultaneously soldered to the electroplated portions.

6. A process for producing separated electrodes I upon the' surface of aceramic body, which comprises providing the entire surface of said bodywith a thin layer of silver, coating those portions of the body surfacewhich are to separate the electrodes with an insulating lacquer,electroplating the exposed portions of the silver layer with copper,removing the insulating lacquer with a solvent, and then dipping theceramic body in a oath of molten solder composed of 8 equal parts byweight of lead and tin, held at about 260 C., until the newly exposedportions of'the thin silver layer have been dissolved by the solder.

. STANLEY O. DORST.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS Name Date Pickard July 21, 1908 1,160, l0Foersterling et al. Nov. 16, 1915 1,359,338 Dagory Nov. 16, 19201,892,755 Scheppmann Jan. 3, 1933 2,062,116 Betterton et al. Nov. 24,1936 2,161,888 Rearick June 13, 1939 2,220,961 Kern Nov. 12, 19402,253,026 Godsey. Jr Aug. 19, 1941 2,398,176 Deyrup Apr. 9, 19462,443,119 Rubin June 8, 1948

1. A PROCESS FOR PRODUCING DISCONTINUOUS, CONDUCTING COATINGS UPON THESURFACES OF INSULATING BODIES, WHICH COMPRISES PROVIDING THE ENTIRESURFACE OF THE INSULATING BODY WITH A THIN LAYER OF A METAL OF THE CLASSCONSISTING OF SILVER, PLATINUM AND PALLADIUM MASKING PORTIONS OF SAIDLAYER WITH AN INSULATING COATING, ELECTROPLATING THE EXPOSED PORTIONS OFSAID LAYER WITH A CONDUCTING METAL OF THE CLASS CONSISTING OF COPPER,NICKEL, CADMIUM, LEAD AND TIN, REMOVING THE INSULATING COATING, ANDDIPPING THE ENTIRE SURFACE IN A BATH OF MOLTEN SOLDER COMPOSED OF ANALLOY OF AT LEAST TWO METALS SELECTED FROM THE GROUP CONSISTING OF LEAD,TIN, BISMUTH, ZINC, COPPER, SILVER AND CADMIUM, UNTIL THE NEWLY EXPOSEDPORTIONS OF THE THIN LAYER HAVE BEEN DISSOLVED BY THE SOLDER.